This invention relates to a method for synthesizing an artificial diamond, and more specifically, to a method for synthesizing a transparent n-type diamond having low resistance in manufacturing a diamond by a vapor-phase growth method, a sputtering method, a high-temperature and high-pressure synthesis method, or the like.
As a method for synthesizing a diamond, a method for artificially synthesizing a diamond from a graphite carbon by using a catalyst under high-temperature and high-pressure (50 kbr, 1500 k or more), and a method for synthesizing a diamond thin film on a substrate by a CVD method from a mixed gas of a hydrocarbon and a hydrogen under low pressure ( less than 1 Torr) and high temperature ( greater than 800xc2x0 C.), or the like, are known. Japanese Patent Unexamined Laid-open Publication No. S62(1987)-70295 discloses a method for manufacturing an n-type semiconductor diamond thin film. According to the method, a reaction gas comprising a gas including P, As or Sb as a dopant element, a hydrocarbon gas and hydrogen is decomposed by heat or decomposed by plasma to be evaporated on a substrate by a microwave plasma CVD method or a thermal decomposing CVD method, to thereby manufacture an n-type semiconductor diamond thin film.
The above-mentioned plasma CVD method and thermal decomposing CVD method have drawbacks such that it is troublesome to execute the method due to the high-temperature processing, the obtained film is easily broken due to the residual stress maintained in the film, and it is difficult to control an amount of dopant. As a method for solving these problems, Japanese Patent Unexamined Laid-open Publication No. H5(1993)-345696 discloses a method for manufacturing a diamond thin film, which is excellent in hardness, corrosion resistance, heat resistance, and the like, and is possible to be applied to a high temperature electric equipment. In the method, a dopant ion beam is simultaneously introduced when a film is growing at low temperature by sputtering an ion beam.
Furthermore, a method for forming a p-type or n-type diamond is also known (Japanese Patent Unexamined Laid-open Publication Nos. H5(1993)-117088 and H5(1993)-117089). In the method, an electron beam or excimer laser is irradiated toward a surface of a single crystal diamond to activate a dopant disposed on the single crystal diamond, so that the dopant is diffused therein. Also known is an n-type semiconductor diamond in which nitrogen atom of 1xc3x971019 cmxe2x88x923 or more is doped (Japanese Patent Unexamined Laid-open Publication No. H7(1995)-69794). Furthermore, it is also known that, in forming an n-type diamond single crystal by a vapor-phase growth method, in order to prevent a reaction of gases due to corrosion thereof before reaching a substrate, each gas is directly supplied in a molecular flow state to the substrate (Japanese Patent Unexamined Laid-open Publication No. H10(1998)-149986).
Furthermore, the inventor invented a method for obtaining a single crystal diamond excellent in crystallization (Japanese Patent Unexamined Laid-open Publication No. H9(1997) -20593). In the method, an amorphous carbon hydride is formed by adding a hydrogen to a carbon, rapidly cooling a decomposed carbon gas hydride on a substrate, or sputtering a graphite by hydrogen atom, and atoms are rearranged into a crystal diamond under low temperature by forming atomic holes and interstitial atom pairs in the amorphous carbon hydride, to thereby effectively cause a movement of the interstitial atom. The single crystal diamond obtained by this method meets characteristics required by various semiconductor materials and optical semiconductor materials requiring a highly controlled crystallization.
(Problems to be Solved by the Invention)
A p-type diamond thin film having low resistance can be easily manufactured by a conventional technique. Although an n-type diamond having high resistance can be manufactured, it is difficult to manufacture an n-type diamond thin film having low resistance because it is impossible to activate at room temperature (300xc2x0 K) due to the self-compensation effect and the deep donor level (500 meV).
If an n-type diamond having low resistance can be synthesized as a single crystal diamond thin film, by combining it with a p-type diamond having low resistance, which is already realized by doping impurities, it is possible to manufacture a high power and high speed semiconductor device using a diamond operable at high temperature and an ultraviolet semiconductor laser diode made of diamond, which is essential for a high density recording and a vast information transmittance.
It is also possible to manufacture a transparent n-type single crystal protective film utilizing high hardness of a diamond, which is excellent in electric conductivity and thermal conductivity. Furthermore, by utilizing the negative electron affinity energy of a diamond, it is possible to manufacture a display having a large surface area made by highly efficient electron beam materials of an n-type diamond having low resistance.
To form an n-type diamond having low resistance means to delete the deep donor level due to N in a synthesized diamond and change the deep level to a shallow level, whereby an absorption of natural light (sun light) is prevented, to thereby extinguish the color caused by the deep level of the single N in the synthetic diamond. This is the same as in forming a transparent synthesized diamond by forming an n-type diamond having low resistance. In synthesizing a diamond by a high-temperature and high-pressure synthetic technique utilizing nickel catalyst or the like, it becomes possible to manufacture a transparent diamond valuable as a jewel.
(Means for Solving the Problems)
The inventor has found the facts that, in order to solve the aforementioned problems, in forming a single crystal diamond thin film on a substrate by a vapor-phase growth method or a sputtering method, by simultaneously doping a p-type dopant and an n-type dopant, it becomes possible to stabilize an n-type dopant in a high densitys, lower an impurity level and greatly increase the number of carriers, to thereby synthesize a high quality single crystal diamond thin film having low resistance.
The inventor has also found the following fact. That is, in synthesizing an artificial diamond by a conventional high-temperature and high-pressure synthesizing method utilizing nickel catalyst or the like, by mixing H as a p-type dopant and N, P or As as an n-type dopant at the atomic density ratio of 1:2 to 1:3 before synthesizing, a donor-acceptor compound, such as a Pxe2x80x94Hxe2x80x94P pair, an Nxe2x80x94Hxe2x80x94N pair or a Asxe2x80x94Hxe2x80x94As pair, is formed in a crystal, to lower the impurity level as compared to a single doping to thereby form an n-type diamond having low resistance. This results in a transparent artificial diamond, wherein a conventional artificial diamond made by a conventional method had color because the natural light is absorbed by the deep impurity level of N.
In other words, the method according to the present invention is based on a principle that, regardless of a method for synthesizing a diamond, by simultaneously doping H as an acceptor and P, N, or As as a donor at the atomic density ratio of 1:2 to 1:3, an acceptor-donor complex (compound) is formed, resulting in a decreased donor level. According to this principle, a diamond in a metallic state (0.001 xcexa9cm) can also be made.
As shown in FIG. 1, by forming a Pxe2x80x94Hxe2x80x94P pair, an Nxe2x80x94Hxe2x80x94N pair or a Asxe2x80x94Hxe2x80x94As pair (complex) by a simultaneous doping, electron scattering due to an n-type carrier dopant is decreased, and the movement of the electron is greatly increased. This lowers the donor level to thereby increase the carrier density in a diamond crystal, which increases the activation rate by 10 to 1000 times, resulting in an n-type diamond having lower resistance.
In the diamond crystal, H as an acceptor and N, P, or As as a donor take a structural position (an impurity complex) forming the crystal model shown in FIG. 2. The positioning of the acceptor atom H between the donor atom (P, N, As) and the donor atom (P, N, As) stabilizes the crystallography structural position. Accordingly, donors can be doped in higher density.
In the method according to the present invention, N, P or As as an n-type dopant in the form of atom and H as a p-type dopant in the form of atom electrically excited by a radio wave, a laser, an x-ray, an electron beam, or the like, are simultaneously doped. Furthermore, a carbon vapor partial pressure, an n-type dopant partial pressure and a p-type dopant partial pressure are controlled so as to increase the n-type dopant atomic density such that the atomic density of the n-type dopant is larger than that of the p-type dopant.
Furthermore, the present invention provides a method for removing H as a donor from the crystal. In the method, a single crystal thin film of the synthesized diamond is cooled once, and annealed at high temperature for a short time in an electric field, so that the donor made of hydrogen is removed from the crystal.
Furthermore, the present invention provides a method for forming a high efficiency spinpolarized electron-beam material. In the method, a circular polarized laser is irradiated to a single crystal thin film of a synthesized diamond.
(Function)
By simultaneously doping an n-type dopant and a p-type dopant, electrostatic energy or lattice energy therebetween is decreased, which stabilizes an n-type dopant and enables a stable doping of the n-dopant in high density, resulting in low resistance.
Furthermore, by simultaneously doping an n-type dopant and a p-type dopant, a pair of an n-type dopant and a p-type dopant is formed in a diamond crystal, which decreases electron scattering of n-type carrier due to an n-type carrier dopant, to thereby increase the carrier movement, resulting in low resistance. In other words, according to the present invention, a diamond single crystal thin film having a film thickness of about 0.01 to about 1 xcexcm and a film resistance of 1.0 xcexa9cm or less can be obtained.